1. Field of the Invention
The present invention relates to an operation mechanism of a variable displacement hydraulic pump in a hydrostatic transmission (hereinafter referred to as an xe2x80x9cHSTxe2x80x9d) adapted as a shift transmission for a vehicle, wherein the operation mechanism is attempted to reduce a shock in shifting the vehicle effectively, to have a good reliability in its motion and to ease its assembling and maintenance.
2. Background Art
As is well-known, there is a conventional HST comprising a hydraulic pump and a hydraulic motor fluidly connected with each other, wherein at least the hydraulic pump is volumetrically variable so that the capacity of the hydraulic pump is varied for changing the traveling speed of a vehicle. The hydraulic pump is provided with a capacity regulating member, e.g., a movable swash plate, operatively connected with a speed control device like a lever or a pedal on a vehicle. The capacity regulating member is operated in correspondence to the operational degree of the speed control device so as to change the traveling speed of the vehicle.
Furthermore, conventionally, there are various well-known means to moderate the operation of the capacity regulating member for avoiding sudden shock in shifting as follows:
Japanese Utility Model Laid Open Gazette No. Hei 3-69,755 discloses a damper provided in an HST housing, wherein a rod of the damper is pivotally connected to a speed control arm for operating the capacity regulating member. Lubrication oil filled in the housing is introduced into the damper. In the damper, the flow of lubrication oil is limited by an orifice or the like. Thus, the damper serves as a fluid-resistive device which uses lubrication oil.
Japanese Utility Model No. Hei 7-16,138 discloses a gas damper replacing the above-mentioned damper, wherein the gas damper using air or the like is disposed in the HST housing, however, out of communication with lubrication oil in the housing.
Japanese Utility Model No. Hei 6-12,318 discloses a damper serving as a fluid-resistive device using HST-operation oil.
However, since the fluid-resistance generated by the damper which uses lubrication oil or HST-operation oil in an HST housing is small, the moderation of the capacity regulating member in motion may be insufficient. On the other hand, the orifice in the damper, if narrowed to increase the fluid-resistance, tends to be choked with dust, thereby preventing the damper from secure action. Furthermore, it must be considered that the viscosity of lubrication oil or HST-operation oil varies so as to change the effect of the damper because the temperature of lubrication oil or HST-operation oil is increased according to operation of the HST.
The gas damper is also desired to be improved because of its small resistance and its large elasticity which causes uncertainty in its damping effect.
Furthermore, the above-mentioned conventional dampers cannot be assembled easily, thereby increasing the number of processes and labor for producing the HST.
An object of the present invention is to provide an operation mechanism which operates a capacity regulating member of a variable displacement hydraulic pump disposed in a housing, wherein the capacity regulating member receives sufficient resistance so as to be moderated in its motion, and the capacity regulating member and the operation mechanism are finally assembled together.
To achieve the object, according to the present invention, the operation mechanism comprises an operating device and a resistive device. The operating device is operationally connected with the capacity regulating member. The operating device receives an operational force from the outside of the housing so as to operate the capacity regulating member. The resistive device is operationally connected with the capacity regulating member so as to give a resistant force onto the capacity regulating member in motion.
The resistive device comprises a casing, a piston slidably disposed in the casing, and fluid hermetically filled in the casing, which is different from fluid filled in the above-mentioned housing for lubrication or operating the hydraulic pump. These component elements are previously assembled or composed together so as to serve as the unified resistive device. The housing is provided therein with a hole, in which the casing of the resistive device is immovably caught with a retainer disposed in the hole. The resistive device as an assembly unit is entirely removable, thereby facilitating its maintenance.
The hole is formed by a center section disposed in the housing. Therefore, the space for arranging the center section is also utilized to arrange the resistive device, thereby contributing to minimization of the device.
Alternatively, the hole may be formed by a wall of the housing so that the resistive device can be easily arranged at a suitable position where it can be actuated without being inhibited by other parts.
The hole may be defined in the housing. Therefore, the resistive device is not exposed outside the housing, thereby being protected from rust and reducing expenses.
The resistive device may be structured by slidably inserting the piston and hermetically filling the fluid into such a hole formed by the wall of the housing or the center section without the casing, thereby reducing the number of component parts.
The fluid of the resistive device which is hermetically filled in the hole or the casing is more viscous than that filled in the housing for lubrication and hydraulic operation of the hydraulic pump. Therefore, sufficient fluid-resistance can be obtained while a restrictive fluid passage, e.g., an orifice, in the resistive device is not greatly narrowed, thereby securing a sufficient sectional area of the restrictive fluid passage so as not to be choked with dust. Consequently, the resistive device can be secured in its actuation and minimized.
Alternatively, the fluid of the resistive device may be less viscous than that filled in the housing for lubrication or operation for the hydraulic pump. Generally, fluid having a little viscosity is less variable in its viscosity than that having a great viscosity regardless of the variation of temperature. Thus, the resistive device is secured in its actuation for moderating the capacity regulating member while the surrounding temperature is increased by operation of the hydraulic pump.
Alternatively or additionally, a resistive device for applying a resistance onto the capacity regulating member of the hydraulic pump in motion may be constituted by a spring provided in a linkage between the operation device and the capacity regulating member.
If this spring-resistive device is provided in addition to the above-mentioned fluid-resistive device, the spring-resistive device may be interposed between the fluid-resistive device and the operating device. Consequently, the capacity regulating member of the hydraulic pump can be more properly actuated by the synergetic effect of the fluid-resistive device and the spring-resistive device.
The spring-resistive device increases its biasing force for the operating device toward its initial position as the operational degree of the operating device is increased. Also, the biasing force is acceleratedly increased when the operational degree of the operating device exceeds a certain value. Therefore, when the operational degree of the operating device is small, the motion of the capacity regulating member is relatively effected by the fluid-resistive device so as to effectively reduce the shock of a vehicle in starting. When the operational degree of the operating device is large, the capacity regulating member is effected by the biasing force of the spring-resistive device rather than the fluid-resistance of the fluid-resistive device, thereby enhancing its response to the operation of the operating device.
Such a spring-resistive device whose biasing force is changed as mentioned above may be constructed simply at low costs as follows:
The spring-resistive device comprises double coiled springs which are a pair of diametrically larger and smaller springs. The free-displacing length of one of the springs is larger than that of the other spring.